Magnetic tapes are classified into L.H. (low noise, high output) tapes, chromium tapes and ferrite chromium tapes, etc. In general, tape recorders are adjusted in accordance with a standard tape among various kind of tapes. However, in tape recorders whose recording conditions are fixed, the recording and reproducing characteristics are greatly fluctuated, because even the magnetic tapes of the same kind are of great difference in characteristics. In each magnetic tape, the frequency characteristic, distortion and M.O.L. (high frequency saturation level) are fluctuated by the variation of the bias current. It is well known in the art that, in general, the high frequency characteristic is upgraded with small bias current whereas it is degraded with large bias current. Furthermore, it is also known in the art that, with large bias current, the distortion factor is low, but the high frequency saturation level is decreased and the dynamic range is lowered, i.e. the high frequency characteristic and the distortion factor are contradictorily affected by the increase or decrease of the bias current. Thus, it can be understood that any magnetic tape has the optimum bias current with which the distortion factor is in balance with the high frequency dynamic range. Accordingly, if the tape recorder is set for this optimum bias current and an equalization characteristic causing the frequency characteristic to be flat is provided, then the characteristic of each magnetic tape can be maximally utilized.